Relationship between heavy marihuana smoking and tests of immune and gonadal function will be studied in man with the following protocol. Experienced male marihuana smokers in good physical and mental health will be hospitalized for 3 months in the Research Ward of the New York Psychiatric Institute under constant surveillance by medical and nursing staff. They will be kept for 4 weeks in a complete drug free state. After this observation period, subjects will be given marihuana cigaretts (20 milligrams THC/cig.) to smoke daily for 1 month. The number of cigarettes smoked will be increased progressively until each subject smokes the maximum number he is will to use. This month of smoking will be followed by a 4 week drug free "wash out period". During the whole period of hospital confinement, venous blood will be sampled once a week. Blood samples will also be drawn at the same time from matched controls who have never smoked marihuana and do not use any medication. All samples will be coded before being sent to the specialized laboratories. During hospitalization, the subjects will also give twice weekly, specimens of semen. In paired blood samples of 5 different concentrations of PHA, Pockweed mitogen and concanavallin A. T rosette Semen volume, sperm concentration, percent motility and sperm morphology will be studied. Test of stimulation of motility by theophyllin will also be performed. Measurements of testosterone, LH and FSH by immunoassay will be made. In view of the decreased sperm count previously observed after one month of heavy marihuana smoking, semen specimens will be examined at weekly intervals for six months after the study, a period of time encompassing 2 cycles of complete spermatogenesis. These studies might delineate the effects of heavy marihuana smoking on the replication of T lymphocytes, production of immunoglobulins, and gonadal function in man.